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Complexes of Carbon Monoxide and Its Relatives An Organometallic Family Celebrates Its Birthday.

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Volume 29 . Number 10
October 1990
Pages 1077- 1176
International Edition in English
Complexes of Carbon Monoxide and Its Relatives:
An Organometallic Family Celebrates Its Birthday **
By Helmut Werner*
Dedicated to Professor Max Schmidt on the occasion of his 65th birthday
Nickel tetracarbonyl was discovered one hundred years ago. Its hundredth birthday deserves
to be celebrated not only by metal carbonyl chemists. The pioneering work of Mond, Langer,
and Quincke has led to developments which, particularly in the last 30 years, have had
consequences in many areas outside that of metal carbonyl chemistry. The parent ligand of the
family, CO, has been joined by a series of relatives which are isoelectronic with it and which
in some cases are even more effective as K acceptors. These are in the main extremely reactive
molecules such as CS, CNH, and C=CH,, which, though very short-lived in the free state,
form very stable complexes with transition metals. This article documents the family relationships by structure and reactivity comparisons; attention is also drawn to the synthetic potential
of the metal carbonyl analogues, which is still virtually untapped. The final section ventures
a look towards the next 100 years, which promise to be just as exciting as the past century.
1. Introduction
An organometailic family celebrates its hundredth birthday this year. In August 1890 an article by Ludwig Mond,
Carl Lunger, and Friedrich Quincke appeared in the Journal
of the Chemical Society with the title “Action of Carbon
Monoxide on Nickel”; in this paper the authors described
the synthesis of nickel tetracarbonyl.”] Although this compound, isolated as a colorless, clear, extremely toxic liquid,
was not the first metal compound containing carbonyl ligands- -the first, [Pt(CO),Cl,], had already been synthesized
by Schiitzenberger in 1868[’’-its
remarkable properties and
its industrial applicability to the preparation of ultrapure
nickel were immediately recognized by Mond. This serendipitous discovery 13] started a development which reached its
[*] Prof. Dr. H. Werner
lnstitut fur Anorganische Chemie der Universitat
Am Hubland, D-8700 Wiirzburg (FRG)
[**I An extended version of a plenary lecture presented at the IVth European
Symposium on Inorganic Chemistry, 12-15 September 1988, Freiburg
Angrw Chem. In!. Ed. Engl. 29 (1990) 1077-1089
first peak in the work of Walter HieberC4’and which, during
the last 40 years, has played a very important role in the
often-cited “renaissance of inorganic chemistry”.
This review will not focus, however, on the chemistry of
metal carbonyls, which remains just as fascinating and as
topical as it has always been (see Fig. 1). Nickel tetracarbony1 and other stable binary carbonyls, some of them already described by Ludwig M ~ n d , [have
~ ] been joined in recent years by very labile carbonyls such as [Cu,(CO),],
[Ag,(CO),], [Pt(CO),], and [Ti(CO),].[’] Thanks in great
part to the pioneering work by Chini, Longoni, Dahl et a1.I6]
and Lewis et al.,17] carbonylmetal clusters have achieved a
degree of importance undreamed of twenty years ago, in
particular as cage frameworks for both smaller and larger
metal and nonmetal atoms.‘*]Moreover, carbonyl metalates
have found an important place in synthetic chemistry-not
only because of the importance of Collman’s reagent,
Na,[Fe(CO),] .[’I There are still unmarked areas on the metal
carbonyl map, however, as shown, for example, by the fact
that the synthesis of salts of the hexacarbonyltitanate dianion, [Ti(C0),lze, the first stable binary carbonyl compound
VerlagsgesellschuJimbH, 0-6940 Wernheim, 1990
0570-0833190/10t0-1077S 3 . 5 0 i .2510
ecule to one or several metal atoms can be either end-on or
via p2 or p3 bridges (Fig. 2). This is also true for the relatives.
The family of CO-like molecules has two separate branches: the first includes the molecules formed from CO by substitution of the carbon atom by a homologous element of the
fourth main group or by an isoelectronic ion (Bo, N@).The
second contains the molecules derived from CO by replacing
p3 bridge
Fig. 2 Coordination possibilities for CO.
Fig. 1 . [Ni(CO),] and other, much later synthesized metal carbonyls
of titanium, was described by Ellis only in 1988, that is,
almost 100 years after the discovery of nickel tetracarbonyl.“’]
The metal carbonyls have meanwhile been joined by “relatives”, which are the subject of this article. Remarkably,
their history is almost as long as that of their “ancestor”.
Soon after the publication of Mond‘s results,“’ Sabatier et al.
in Toulouse attempted to bond NO, NO,, C,H,, and C,H,
to nickel and similar metals; although they were unsuccessful, their attempts did in fact lead to the discovery that nickel
could be used as a catalyst in hydrogenation.{”] There was
then a gap of 75 years before Wilke et al. were successful in
preparing [Ni(C,H4)J ,[I2] a compound similar to [Ni(CO),]
although structurally different. It was then used in the synthesis of further nickel(o) complexes, some of which are catalytically extremely active.” 31
Here, however, ethylene will not be regarded as a relative
of carbon monoxide. Although there are indeed close relationships between metal carbonyls such as [Ni(CO),] and
compounds such as [Ni(C,H,)3],[’21 [Ni(PC13),],1’41[Ni(PPh3),],[’ and K,[Ni(C E CH)4],[’61only those molecules or
fragments that are valence-isoelectronic with CO will be considered relatives. Such ligands resemble CO not only formally but also in their coordination properties and thus form a
family with it. For example, the bonding of the parent mol-
the oxygen atom by S, Se, or Te or by an isoelectronic fragment such as NH or CH,. The corresponding classes of
compounds will not be discussed in detail; instead, this article will make clear the relationships within the family as well
as the particular features of the various members of the
2. The Relatives of the Type [XEO]:
Known and Unknown Examples
The closest relative of CO, at least from the point of view
of the preparative chemist dealing with this type of molecule,
is surely SiO (Scheme 1). This molecule is formed when SiO,
is heated with Si under vacuum at 1250 “C and can be detected both in the gas phase and at low temperatures in a suitable
matrix. The Si-0 bond length of 1.51 8, agrees well with the
theoretical value of 1.50 8, and shows that SiO is isosteric
with CO.[”]
p Z T
5-0 1.51 8,
N-o 1.06a
Scheme 1.
Helmut Werner was born in 1934 in Miihlhausen (Thiiringen). He studied chemistry at Jena,
where he received his “Diplom” under Franz Hein, and at the Technische Hochschule in Munich,
where he received his doctorate under E. 0. Fischer. After postdoctoral research under J: H.
Richards at the California Institute of Technology in Pasadena (1962- 1963), he completed his
“Habilitation” at the end of t966 on kinetic investigations of substitution reactions of
organometallic complexes. In 1968, he began work in inorganic chemistry at the Universitat
Zurich, where he became afullprofessor in 1970. At the end of 1975, he moved 10 the Universitat
Wiirzburg. Helmut Werner has received numerous prizes and honors, including the Alfred-StockGedachtnispreis of the German Chemical Society: since 1988, he has been a full member of the
Deutsche Akademie der Naturforscher Leopoldina.
Angew. Chem. Inl. Ed. Engl. 29 (1990) 1077-1089
[Cr( NO141
c l o
Fig. 3. The [M(CO),(NO),. .]family (n
1.84 I
[Ni t C 0 l 4 ]
Metal complexes with SiO as a ligand are still unknown.
Although it has been possible during the last 30 years, starting with the synthesis of the first metal cyclobutadiene complexes by Criegee et a1.[181and Hiibel et al.,“91to bind many
molecules that are extremely short-lived in the free state
(“nonexistent” molecules) within the coordination sphere of
a metal and thus to investigate them, corresponding attempts to isolate or detect complexes of the type [M(SiO)L,]
have so far been unsuccessful. Even matrix studies have provided no evidence for their existence.lZo1
The situation with regard to the anion BOe is similar. The
protonated molecule HBO has been detected experimentalI Y ; [ ~ ’ I however, salts derived from it such as MBO (M = Na,
K, etc.) still do not exist. Rosmus[’’] recently showed from
SCF calculations that the potential energy functions of KBO
and KCN are very similar, so that the molecule would probably be quasi-linear. This structural prediction also applies
to methyl boroxide, which is formally derived from BOe by
methylation; this molecule was first prepared as an intermediate by Paetzoldet al.Iz3]and was characterized very recently by its PE spectrum.[z41In view of the vast scope of the
chemistry of metal carbene complexes [M( = CXY)L,] it
seems remarkable that only very little is known about comparable (isoelectronic or isosteric) compounds of the type
[M’(= BXY)L,].‘251
The picture becomes completely different on moving to
the right of CO, that is, to NO@.Not only are many salts of
this cation known (the N-0 distance of 1.06 8, corresponds
exactly to that of a triple bond, as in CO) but a great deal is
also known about its coordination properties.[z61In fact, the
first nitrosyl complex, the dication [Fe(NO)(H,O),]’@, was
discovered in 1790 by Priestley! Although carbonyl and nitrosyl metal compounds have many similarities, very little is
known about species of the type [M(NO),] or about the
corresponding cations or anions. Only [Cr(NO),] has been
characterized: Herberhold and Razavi obtained this complex
as a brown, low-melting solid by photolysis of [Cr(CO),] in
an NO atmosphere.[”] The species [Co(NO),] has also been
reported ;[’*I however, clear evidence for the existence of this
compound is still wanting. Binary metal nitrosyl clusters are
still an unknown class of substances; they would be of general interest in connection with the bonding of NO at metal
Mixed carbonyl-nitrosyl metal complexes are better
known. As early as 1922, Robert Mond (the son of Ludwig
Mond) et al. described the compound [CO(CO),(NO)];‘’~~
Hieber et al. and later Lewis et al. prepared the isoelectronic
molecules [Fe(CO),(NO)2][301and [Mn(CO)(NO),]
formal analogy between such compounds, which was later
Angew. Chern. Int. Ed. Engl. 29 l1990) 1077-1089
confirmed on a structural basis (Fig. 3), prompted See1 in
1942 to propose the “nitrosyl shift relationship”,[3z1which is
still of importance today as a heuristic principle. According
to this postulate, it is possible to replace Ni in [Ni(CO),] by
Co when one of the CO ligands (two-electron donor) is
simultaneously replaced by the three-electron donor NO.
The metal atoms in the ML, series (Fig. 3) thus act as
“pseudonickel atoms”. Only one analogue of iron pentacarbonyl, the manganese complex [Mn(CO),(NO)] ,[331 has
been described; the remaining members of the ML, series,
[cr(co>,(No),l, IV(CO),(NO),l, and [Ti(CO)(NO),I still
remain to be discovered.
NO0 is a stronger 71 acceptor than CO, as can be seen both
from the stretching vibration frequencies in the IR spectra
and from the bond distances determined by Hedberg and
c o - w o r k e r ~ [(see
~ ~ ’ Fig. 3). In particular, the differences in
the Fe-C and Fe-N bond distances in the compound
[Fe(CO),(NO),] confirm that the nitrosyl ligands are more
strongly bound; this is also documented by the easier exchange of CO (e.g., for PR,) in substitution reactions of
carbonyl-nitrosyl metal complexes.
Although, as already mentioned, binary metal nitrosyl
clusters do not yet exist, bi- and polynuclear compounds
with NO as a ligand are already known;[26]they show the
same possibilities for coordination of NO (or, to be more
precise, NOe) as for CO in the corresponding metal carbony1 complexes (see Fig. 4). When cyclopentadienyl is used
p2 bridge
,u3 bridge
Fig. 4. Coordination possibilities for NO.
as the coligand, an end-on (linear) M-N-O arrangement is
found in [CpNi(NO)] /351 whereas p, and F, bridges exist in
[Cp3Mn3(12-N0)3(p3NO)] /371 respectively. The structural relationship with CO is
extremely obvious here.
Until a few years ago almost nothing was known about
analogies in reaction behavior between CO and NO in the
coordination sphere of a metal. In particular, it was unclear
whether one of the most typical reactions of coordinated
CO, insertion into a M-C bond in the presence of an alkyl or
aryl ligand (which is also the key step in important catalytic
processes such as the 0x0 process or the Monsanto acetic
is also possible for NO. Bergman et al.[391
were the first to show that an alkyl(nitrosy1)metal complex,
formed in the same way as an alkyl(carbony1) compound by
alkylation of the corresponding anion, reacts with PPh, to
give a nitrosoalkane derivative via N-C bond formation
(Fig. 5). The coordination number of the metal remains un-
The extreme tendency of CS to polymerize makes it understandable that so far almost all attempts to synthesize thiocarbonyl metal complexes fromfree CS and a metal or metal
compound have been unsuccessful. Only the cocondensation
of CS with nickel atoms in an argon matrix at 10 K gives a
product whose IR spectrum indicates it to be [Ni(CS),] .[461
In contrast to [Ni(CO),], it is unstable at room temperature,
decomposing to give a black solid which is difficult to characterize.
In the case of iron and chromium, whose carbonyl complexes [Fe(CO),] and [Cr(CO),] are much more stable than
mi(CO)4J, the corresponding binary metal thiocarbonyls
[M(CS),] are still unknown. Mixed carbonyl-thiocarbonyl
complexes are however known (Fig. 6). In the 1970s Angelici
Fig. 5. NO insertion. R = alkyl
changed. In a very similar manner the same group has synthesized and structurally characterized the complex
[C,Me,Ru(N(O)Et)(Ph)PMe,Ph] starting from PMe,Ph
and [C,Me,Ru(NO)(Et)Ph] .1401 On the basis of mechanistic
studies,[39b1there is no doubt that the “NO insertion” is in
fact an alkyl migration to the nitrosyl ligand, as is the case
for the “CO insertion”.[41]Legzdins et al.[421and VolZhardt et
al.[431have shown very recently that this process occurs not
only with neutral donors such as tertiary phosphanes but
also with NO@itself.
3. The CO Homologues CS, CSe, and CTe:
Unstable Molecules with a Rich Coordination
Whereas CO and CO, are both thermodynamically stable,
for the homologous sulfur compounds this is only true for
CS,. The molecule CS, which can be formed from CS, by
photolysis, thermolysis, or cold electrical discharge, is stable
only below - 160 “C (Scheme 2) and polymerizes very rapid-
stable <-16O’C
Scheme 2.
ly at higher temperatures ;[441 this process can occur in an
explosive manner. In spite of its extreme reactivity, the spectroscopic data for CS are known very precisely, and clear
evidence on the mechanism o f its formation from CS, is
available.r44c1Much less is known about CSe,[451and practically nothing about CTe; the latter has so far resisted all
attempts to detect it, so that it still belongs to the group of
“nonexistent” diatomic molecules.
Fig. 6. Metal carbonyls and mixed CO-CE metal complexes.
et al. and Butler et al. synthesized the [Cr(CO),] analogues
[Cr(CO),(CS)] and [Cr(CO),(CSe)],r4’1 and somewhat later
Petz reported the preparation of the [Fe(CO),] analogue
[Fe(CO),(CS)] .[481 These complexes, which are stable under
normal conditions, are volatile and can even be handled in
air; their other properties are also very similar to those of the
parent compounds [M(CO),].
The characterization of a complete series of the general
composition [M(CE)L,] with E = 0, S, Se, and Te was carried out by Roper and co-workers, who obtained the complexes [OsCl,(CE)(CO)(PPh,),] by reaction of [OsCl,(CCI,)(CO)(PPh,),] with EH@.f491 Exact structural data are available for all the members of the series; [OsCl,(CTe)(CO)(PPh,),J was the first metal tellurocarbonyl compound to be
Since free CS, CSe, and CTe are not available as starting
materials, it is necessary to find suitable precursors for the
synthesis of complexes with carbon monochalcogenides as
ligands. As is shown in Figures 7-9 for CS, three routes for
the preparation of compounds of the type [M(CS)L,] have so
far proved generally applicable.[501In each case, the thiocarbony1 ligand is generated at the metal and remains without
exception strongly bonded to it.
The oldest and with hindsight most obvious synthetic
method starts from CS,; its discovery is, so to speak, a “byproduct” of the trailblazing work of the Wilkinson school on
the catalytic abilities of [RhCl(PPh,),] . When studying the
behavior of this compound in various solvents, Baird and
Wilkinson noticed that it reacted with CS, to give the squareplanar complex trans-[RhCl(CS)(PPh,),] via an unstable intermediate with a presumably a-bonded and a 0-bonded CS,
ligand.f5’] In a similar manner (see Fig. 7) Butler and coworkers were able several years later to obtain the manganese thiocarbonyl compound [CpMn(CO),(CS)] ,which is
Angew. Chem. Int. Ed. Engl. 29 (1990) 1077-1089
homologous to [CpMn(CO),], and also to isolate the intermediate [CpMn(CO),(q2-CS,)] .Is2’ It reacts with PPh, to
give SPPh, and [CpMn(CO),(CS)]; presumably, the phosphane attacks the sulfur atom bound to the metal and not the
exocyclic sulfur atom. The same principle has been applied
to prepare further thiocarbonyl and selenocarbonyl complexes (the latter starting from CSe,),[501whereby the isolation of [CpMn(CO)(CS),] as the first compound with two CS
ligands deserves particular
CI \ Rh/pph3
A third method for the synthesis of CS complexes starts
from heteroallenes such as CSSe (Fig. 9). h t h e course of our
initial work on the coordination chemistry of CS,, we found
that the compounds [CpM(PMe,)(q2-CS,)] (M = Co, Rh)
are formed readily and in good yields starting from [CpCo(PMe,),] and [CpRh(PMe,)(C,H,)]; however, the former
do not undergo sulfur abstraction with PR, .ls5] At the same
time we observed that the reaction of [CpCo(PMe,),] with
COS gives the carbonyl complex [CpCo(PMe,)(CO)], so
that we expected that reaction with CSSe would form the
thiocarbonyl compound [CpCo(PMe,)(CS)] . This is in fact
the case, although, depending on the reaction conditions, its
formation may be accompanied by that of the corresponding
thiocarbonylselenide complex [CpCo(PMe,)(q 2-CSSe)]
The latter reacts quantitatively with PPh, to give [CpCo(PMe,)(CS)]. In the case of rhodium, the conversion of
[Rh](C,H,) to [Rh](q2-CSSe) and [Rh](CS), where [Rh] =
[CpRh(PMe,)], can be readily carried out stepwise; here, as
for the case where cobalt is the central atom, the reaction of
[Rh](q’-CSSe) with PPh, leads solely to [Rh](CS) and not to
Fig 7 Synthesis of metal thiocarbonyl complexes from CS,.
The second likely source for thiocarbonyl ligands is
thiophosgene. As shown in Figure8, its reaction with
[Fe(C0),lze and [M,(C0),o]2G (M = Cr, Mo, W) gives the
complexes [Fe(CO),(CS)] and [M(CO),(CS)], although the
yields are not particularly
Another synthesis of
[Cr(CO),(CS)] starts from the metal arene compounds
[(C,H,R)Cr(CO),(CS)] (R = H, CO,Me), which can be
formed from [(C,H,R)Cr(CO),(thf)] and CS, ; the aromatic
ring ligand is then displaced by C0.147b1
=‘Pd, P I
1 E = S , Se
Fig. 9. Synthesis of metal thiocarbonyl complexes from heteroallenes.
SCCI, reacts with square-planar rhodium(1) and iridium(1)
complexes such as [RhCI(PPh,),] and [IrCI(N,)(PPh,),] via
oxidative addition to give the corresponding thiocarbonylrhodium(rr1) and -iridium(IrI) compounds.[531The presumed
intermediate contains the unit M[C( = S)Cl](Cl), which
forms the stable final product via a 1,2 shift of chlorine from
carbon to the metal. Treatment of [Ru,(CO),,] with thiophosgene followed by fragmentation of the cluster results in
a similar reaction, leading to [RuCl,(CS)(CO),]
A n g m Chem h i . Ed. Engf 29 (1990) 1077- I069
A fragmentation of coordinated CS, to give CS and S or
of CSSe to give CS and Se can be caused not only by PR, but
also by nucleophilic metal complexes. In earlier studies we
had already observed that the cobalt compound [CpCo(PMe,)(q2-CS,)] reacts with the metal base [CpCo(PMe,),]
to give the trinuclear cluster [(CpCo),(p,-CS)(p,-S)] in almost quantitative ~ i e l d . 1 ~A~similar
metal-initiated cleavage of CS, or CSSe takes place when either [Pt(PPh,),] or
[Pt(C,H,)(PPh,),] is allowed to react with the chelate phosphane complexes [(L-L)M(qz-CSE)] (M = Pd, Pt; E = S,
Se; L-L = Ph,PC,H,PPh,,
o-C,H,(CH,PPh,),).[581 The
products (Fig. 9) are the binuclear sulfido- or selenidobridged thiocarbonyl compounds [(L-L)M(p-E)Pt(CS)(PPh,)]; it seems likely that the complexes [(L-L)M
(p-CSE)Pt(PPh,),] are formed as intermediates. In the case
where L-L = o-C,H,(CH,PPh,),, M = Pd or Pt, and E = S
or Se, products with this composition can be
Surprisingly, the addition of the ligands Ph,PC,H,PPh, or
o-C,H,(CH,PPh,), to the complexes [(L-L)M(p-E)Pt(CS)(PPh,)] leads not only to the displacement of PPh, but also
to a recombination of the fragments CS and E to give CSE,
so that the final product is an unsymmetrical binuclear complex of the type [(L-L)M(p-CSE)Pt(L-L)] with two chelating
phosphane ligands. There are no other known examples
of such a process (fragmentation of a triatomic molecule
and its re-formation under the influence of the same
metal). The thermolysis of [(PPh,),Pt(p-CS,)Pt(PPh,),]
and [(L-L)Pt(p-CS,)Pt(PPh,),] (L-L = o-C,H,(CH,PPh,),)
leads to the formation of the compounds [Pt(CS)(PPh,),]
and [Pt(CS)(L-L)]; these are the first mononuclear thiocarbonyl complexes of plat in urn(^).[^^^
There are only small differences between the modes of
coordination adopted by CS and CO. As shown in Figure 10,
p 2 bridge
and [(CpCo),(p,-S){ p,-CSCr(CO),)], respectively) [571 by X-ray structural analyses. Thus, CS is considerably
superior to the parent molecule CO with respect to this additional bonding type and offers a synthetic potential for heterometal multinuclear complexes which has so far hardly
been used.
4. Isocyanides as Ligands: CO Analogues
with Advantages and Disadvantages
The molecule CNH is strictly isoelectronic with CO; like
CS, it is extremely labile in the free state, isomerizing very
rapidly to HCN.[661However, it can be readily generated in
the coordination sphere of a metal, where it is stable (a
further analogy to CS). The formation of a CNH ligand can
be carried out most simply by protonation of anionic cyano
complexes; for example, treatment of [M(C0),(CN)le with
HCI or [(C,H,R)Mn(CO),(CN)]@ with H,PO,: in this manner, the neutral compounds [M(CO),(CNH)] (M = Cr, Mo,
W)[671and [(C,H,R)Mn(CO),(CNH)] (R = H, Me)[,’] are
formed. Both the spectroscopic properties and the behavior
of the complexes [M(CO),(CNH)] (M = Cr, W) leave no
doubt that the M-CNH bond is considerably stronger than
the M-NCH bond in the corresponding isomers
[M(CO),(NCH)] , which are formed by allowing
[M(CO),(thf)] to react with HCN.[67b1
p3 b r i d g e
Scheme 3
Fig. 10. Coordination possibilities for CS
the thiocarbonyl ligand can be coordinated both end-on or
in a bridging manner (between two or three metal atoms); it
should be noted that, when CO and CS are both present in
the same molecule (as, for example, in [Cp,Fe(CO),(CS),][601),the formation of CS rather than CO bridges is
CS is a stronger n acceptor than CO; this is shown not
only by MO calculations[611but also by the structural data
of mixed carbonyl thiocarbonyl metal complexes. For example, the Cr-CS distance in [(C,H,CO,Me)Cr(CO),(CS)] is
shorter by 0.05 8, than the Cr-CO distance (mean value)[621,
and a similar difference is observed in the cation [Ir(C0),(CS)(PPh,),le (as the PF, salt).[631This behavior indicates a stronger synergistic interaction between the metal
and the thiocarbonyl ligand. Interestingly, the sulfur atom is
clearly nucleophilic in all coordination modes of CS; it can
not only undergo alkylation but also add 14-electron fragments such as [Cr(CO),] or [CpMn(CO),]. The resulting
complexes have a bent arrangement for (M),C-S-M’, the
exact structure of which has been determined for both n = 1
(e.g., in [(C,H,Me)Cr(CO),CSCr(CO),][641)
and n = 2 and
3 (e.g., in [Cp(PMe,)Co(p-CO){p-CSMn(CO),Cp)Mn(CO)1082
Alkyl and aryl isocyanides, which can be prepared in various ways, are much more stable than the parent compound
They are generally stronger D donors than CO;
as a consequence, numerous binary (“homoleptic”) cationic
complexes can be isolated as the corresponding salts (see
Table 1): examples are [M(CNR),]’@ (M = Cr, Mo, W),
[Ni(CNR),IZ@, [Ag(CNR),]@, and
[CO,(CNR>,,,]“~.[”~Comparable carbonyl metal compounds are not yet known.
Table 1. Metal isocyanide complexes of 3d transition metals
Like CO, isocyanides (CNR) are good .n acceptors; not
only have various metal(o) complexes of the type [M(CNR),]
( n = 4 , M = C r , Mo, W; n = 5 , M = F e , Ru, 0 s ; n = 4 ,
M = Ni) been synthesized,[711but also very recently the first
anionic complex, [Co(CNR),I0 (R = 2,6-Me,C,H,)[721 (i.e.,
an analogue of [Co(C0),le). Binuclear complexes such as
[Co,(CNR),] and [Fe,(CNR),], which resemble the correAngeew. Chem. lnr. Ed. Engl. 29 (1990) f077- 1089
sponding metal carbonyls not only in molecular formula but
also structurally, are also known.[71iAs a historical reminiscence, we may mention that the most direct relatives in this
series to the parent Ni(CO),), the compounds [Ni(CNR),]
(R = C,H,, p-EtOC,H,), were obtained exactly 40 years
ago in the same city (Munich) by two groups working fully
independently of one another, in both cases via ligand substitution reactions starting from [Ni(CO),] .[731
In the case of L = CNR, however, this synthetic method
(formation of [ML,] from [M(CO)J and L) is extremely unusual and can only be used in the absence of a catalyst when
a labile starting material is available. Under purely thermal
conditions, the reaction of metal hexacarbonyls, [M(CO),J
(M = Cr, Mo, W), and of iron pentacarbonyl, [Fe(CO),],
with isocyanides often leads only to monosubstitution even
at higher temperatures and with long reaction times.[71e1
use of phase-transfer catalysts can improve the situation.[74]
However, it is even more favorable to carry out the reaction
of [M(CO),J or [Fe(CO),J with CNR in the presence of a
heterogeneous catalyst such as CoCI,, activated carbon, or
metallic platinum on an oxide support. This method, develhas made it possible to
oped by Coville and co-w~rkers,[’~]
replace all the CO ligands in the otherwise inert Fe(CO), by
CNR and also to substitute more than half of the carbonyl
groups in metal clusters such as [Ir4(CO)12].Apart from
metal carbonyls, arene and cyclodi- and -triene complexes
have been used as starting materials for reactions with isocyanides; the synthesis of [Cr(CNR),] from bis(naphtha1ene)chromium and CNR (R = nBu, C,H 1) is a good
The coordination properties of CO and isocyanides are
similar. The most generally encountered bonding situations
are compared in Figure 11, top; it should be mentioned that,
p2 bridge
Fig. 11. Coordination possibilities for CNR.
Angew Chem. I n t . Ed. Engl. 29 (1990) 1077-1089
in some cases, terminally bonded isocyanide ligands can be
bent at the N
The ligand CNR can also bridge two or three metal atoms,
in the manner shown in Figure 1 1, bottom. A good example
of the p2-C,N form is the nickel cluster [Ni,(CNtBu),] deA comparable interaction of
scribed by Muetterties et
CO with two or three metallic centers has been discussed for
the adsorption of carbon monoxide at surfaces ;[78i the possible threefold bridging seems particularly likely when M is a
transition metal and M’ an electropositive metal such as K,
Mg, or Al.
CO and CNR are also closely analogous in their reactions.
Both isocyanide and carbonyl ligands are able to undergo
insertion into M-C (3 bonds; this process can take place
either spontaneously or in the presence of a Lewis base L (see
Fig. 12). Whether the acyl or imidoyl residue formed is
bonded in a mono- or bidentate manner depends ultimately
on the electronic configuration and the required coordination geometry of the metal.
Fig. 12. Insertion of CO and CNR into M-C
There is one interesting reactivity difference between
structurally analogous acyl and imidoylcobalt complexes of
the half-sandwich type formed by CO and CNR insertion,
respectively (see Fig. 13). While the compound [CpCoMe(CO)(PMe,)]I, obtained from [CpCo(CO)(PMe,)] and methyl iodide in toluene, undergoes only isomerization to [CpCo{C(O)Me)(PMe,)I] when acetone is added,[791a cycloaddition reaction takes place when the corresponding methyl
isocyanide complex is dissolved in this solvent. A five-membered metallaheterocycle is formed, probably via an intermediate with the partial structure Co-C(Me) = NCH,; the final
product has been characterized by an X-ray structural analysis.[801Furthermore, [3 + 21 cycloaddition reactions occur
with acetaldehyde and benzaldehyde as well as with a number of nitriles R C N (R’ = Me, Ph, cyclo-C,H,, CH = CH,,
CMe=CH,, CH=CHMe, NH,, NMe,, SMe); the fiver
membered ring of the type [Col-C(CH,) = NR-CR = N
shown in Figure 13 is thermodynamically unstable and isomerizes to [CoJ-C(= CH,)-NR = CR’-NH, independent of
the nature of the group R.[811With CS, it is even possible to
realize a double cycloaddition, leading to the dimetallaspiroheterocycle depicted in Figure 13; this compound has been
structurally characterized.[*’] The difference in behavior between coordinated CO and CNR documented by this and
other resultsr831is probably due to the basicity of the CNR
nitrogen atom being higher than that of the oxygen in CO;
moreover, the building block [MI-C(Me) = NR formed by a
methyl shift is a better 1,3 dipole than the corresponding unit
[MI-C(Me) 0.
,o"' 7
Fig. 13. Reactivity of [CpCo(CO)(PMe,)] and the CNR analogue
The greater polarity of the C-NR bond compared with
the C E O bond causes metal isocyanide compounds to be
suitable starting materials for the formation of aminocarbene complexes. The pioneering work in this area was done
by C h a f f and ~o-workersI~~1,
who showed that platinum(n)
isocyanide compounds of the type cis-[PtX,(L)(CNR)]
(X = CI, Br, I; R = Me, Et, Ph, p-Tol; L = PMe,Ph, PEt,,
PEt,Ph) react with primary alcohols and amines even under
very mild conditions to give cis-[PtX,(L)(C(NHR)(OR'))]
and cis-[PtX,(L)(C(NHR)(NHR')}], respectively. Somewhat later, this synthetic principle was extended to isocyanide complexes of iron,@']
rhodium,[881ruthenium,[891osmium,[901and gold;[911
a large
number of carbene complexes of these metals could thus be
The observations (confirmed by kinetic studies)
that both primary and secondary amines react faster than
alcohols and that electron-withdrawing groups R at the isocyanide nitrogen atom accelerate the reaction indicate that
the formation of the carbene ligands C(NHR)(NHR) and
C(NHR)(OR') occurs via an initial attack of the amine or
alcohol at the carbon atom of the isocyanide.
- X-P<=C=NR
N H R'
M =C
Fig. 14. Nucleophilic addition to CNR and CO metal complexes.
Like CNH, vinylidene (CCH,) is in a strict sense isoelectronic with CO; in addition, it is similarly labile in the free
state, in analogy to its nitrogen relative. The energy barrier
for the conversion of C=CH, to its isomer acetylene
HC = CH has been calculated by Schaeffer et al. to be only
ca. 4 kcal mol-' and could in fact be even lower because of
zero point energy effects.[951The lifetime of C=CH, can
thus be estimated to be lo-" to 1 0 - l 2 s ; this is in good
agreement with the results of trapping experiments.[961
5. The Completion of the Series CO, CNH, CCH,:
Vinylidenes as Building Blocks for Metal Complexes
A similar conversion of a coordinated carbonyl group to
an amino(hydroxy)carbene ligand C(OH)(NHR) or
C(OH)(NR,) is not known; the nucleophilicity of a primary
or secondary amine is apparently not sufficient to make this
possible. However, Fischer and Koiimeier showed almost
simultaneously with the work carried out by Chatt and coworkers (see Fig. 14) that lithium dialkylamides do undergo
nucleophilic attack at a CO group bound to a metal (e.g., in
[Cr(CO),]); subsequent alkylation gives an alkoxy(amin0)carbene ligand.[931This variation of the classical Fischer
method for the formation of metal carbene complexes has
found further application[92] and has proved useful in organic synthesis.[94]
In contrast to isocyanides (CNR), vinylidene derivatives
of the general type C = CHR and C = CR, are also unstable
and thus not available as starting materials for synthesis of
complexes. It is therefore absolutely necessary to generate the
ligand C = CH,, C = CHR, or C = CR, at the metal; as will
be shown below there are various methods for doing so.
The first results in this direction were obtained only two
years after the discovery of the carbene complexes; again, as
so often, luck played an important role. Mills and Redhouse[97]had attempted to prepare an iron carbene complex
by irradiating a benzene solution of [Fe(CO),] and diphenylketene; however, they obtained a product with the molecular
formula [Fe,(CO),(C,Ph,)J, the X-ray structural analysis of
which showed it to be a bridged vinylidene complex. The first
mononuclear compound with a vinylidene Iigand was deAngew. Chrm. I n [ . Ed. Engl. 29 (1990) 1077-1089
scribed in 1972 by King and Saran, who isolated the compounds [CpM{C=C(CN),)L,Cl] (M = Mo, W ; L = PPh,,
AsPh,. SbPh, ,etc.) in the course of studies on the reactivity
of 1-chloro-2,2-dicyanovinyl complexes of molybdenum and
tungsten.[981The extremely high thermodynamic stability of
the vinylidene-metal bond observed by them has also been
confirmed for many other examples; it is thus not surprising
that today vinylidene complexes of almost all the transition
metals are known.[991
Very exact information on the structures and bonding
modes of these complexes is available. As can be seen from
the comparison in Figure 15, not only vinylidene and carbony1 complexes but also vinylidene and isocyanide com-
only protonation but also alkylation [using MeOS0,F or
[ORJBF, (R = Me, Et)]; the yields are practically quantitative, indicating that the 0-C atom of the alkynyl ligand and
not the metal is the preferred site for attack by the electrophile (even for L = PMe,). The observation that anionic
alkynyl compounds can undergo electrophilic addition reactions even more readily than neutral compounds corresponds to expectations; it has been used by Berke and coworkers, in particular, for the synthesis of the vinylidene
manganese complexes [CpMn(CO),(C = CRR')] .['
M =
Fe, Ru
M = Mn, Re
Fig. 16. Synthesis of vinylidenemetal complexes from alkynylmetal compounds and 1-alkynes.
Fig. I S . Coordination possibilities for C =CH,
plexes show common structural features. The vinylidenes as
terminally bonded ligands have excellent r-acceptor properties, which according to a comparative spectroscopic investigation are better than those of CO and only a little worse
than those of CS and SO,.['oo1 It is remarkable that the
rotational barrier for the M = C bond is very small for both
isocyanide and vinylidene
so that optical isomers have not yet been detected for allene analogues of the
type [LL'M = C = CRR'] .Ii
A large number of compounds with doubly bridging
vinylidene ligands are known; these can be obtained both
directly (see the iron complex [Fe,(CO),(p-C = CPh,)] already mentioned)[971and stepwise via addition of a coordinatively unsaturated metal compound to an M = C = C R R
unit."91 The latter method is important for the formation of
binuclear complexes with two different metal atoms. The
p,-C,C bridges (Fig. 15) are also by n o means unusual and
are found in particular in ruthenium and osmium trinuclear
clusters.[' 031
The aforementioned extraordinary lability of the compounds C = CRR' leads naturally to the question how complexes containing the structural unit M = C = CRR' can be
prepared. Two methods have so far proved generally useful;
these start either from alkynylmetai compounds or from coordinatively unsaturated or substitution-labile metal complexes and I-alkynes. Other methods, which employ vinylidene sources such as ketene~,~'~'
1,I -dichloroolefin~,['~~]
diazoolefins,[' 06] and a-(chlorovinyl)silanes,[' 0 7 ] can be used
in special cases but have not attained general importance.[99]
The road to vinylidene complexes starting from alkynylmetal compounds was opened in particular by Davison and
co-workers['081and Bruce and c o - w o r k e r (see
~ ~ ~Fig.
~ ~16).
Cyclopentadienyl iron and ruthenium compounds of the
general composition [CpML,(C= CR)) can undergo not
Angeu Chem In1. Ed. Engl. 29 (1990) 1077-1089
The pioneering work on the conversion of the thermodynamically much more stable I-alkynes (HC=CR) into the
isomeric vinylidenes (C = CHR) in the coordination sphere
of a metal was done by Antonova, Kolobova, and their coworkers.['00*'''I They found that the photochemical reaction of [CpM(CO),] (M = Mn, Re) with phenylacetylene
does not give the expected alkyne metal compounds [CpM(C0),(q2-PhC = CH)] but instead the isomeric vinylidene
complexes [CpM(CO),(C = CHPh)]. The IR spectra recorded during the reaction indicated that the alkyne is first coordinated, but attempts to isolate the intermediate [CpM(CO),(q'-PhC-CH)]
in an analytically pure form were
"'I. Similar results were obtained later in
the reactions of the systems [CpML,]@ (M=Ru, Os),
[Cr(CO),], and ~(CO),(R,PC,H,PR,)] (all of which
are related to [CpM(CO),] (M = Mn, Re)) with 1 -alkynes
reported by Bruce,['ogb- 21 Berke," ' 31 and Templeton[' 14]
and their co-workers. In each case vinylidene rather than
I-alkyne complexes were formed.
The question thus raised regarding the mechanism of the
conversion of 1-alkynes to metal-bound vinylidenes could at
first be answered only in a speculative manner. The suggestion made by various authors, in particular by Antonova and
co-workers[' ''I (see Fig. 17), was that subsequent to the coordination of the alkyne an intramolecular oxidative addition takes place, the alkynyl(hydrido) intermediate then un-
Fig. 17. Proposed mechanisms for vinylidene formation from I-alkynes.
dergoing a 1,3-H shift to give the vinylidene complex. Although this appeared plausible, there was at first no experimental evidence for it. In addition, a very detailed study by
Silvestre and Hofmann[1151 showed that the isomerization
of [L,MH(C = CR)] to [L,M(C = CHR)] would have a very
high activation energy, so that the stepwise conversion of the
alkyne to the vinylidene complex shown in the upper part of
Figure 17 would be improbable. They suggested that a slippage of the I-alkyne into a position in which only one carbon
atom of the triple bond interacts with the metal should be
more favorable; this is reminiscent of the bonding situation
in a vinylcarbenium ion (see Fig. 17).
Evidence that a stepwise interconversion is possible after
all and that, with the correct choice of ligands, the three
possible isomers can be isolated was produced in the course
of our own work on the reactivity of coordinatively unsaturated triisopropylphosphane rhodium and iridium complexes. In the initial stages we were able (see Scheme 5 ) to trans-
Scheme 5. L
The final reaction products shown in Schemes 5 and 6
behave in part very similarly to their CO relatives. For example, reaction of both the compounds trans-[IrCl(C = CHR)(PiPr3)2] and the Vaska complex tran~-[IrCl(CO)(PPh,)~]
with HBF, results in protonation at the metal,['201whereas
the half-sandwich compounds [CpRh(C = CHR)(PiPr,)]
(R = H, Me, Ph) and [CpRh(CO)(PMe,)] react with benzoyl
azide to give the same type of five-membered metallaheterocycIes.['211
The parallels in reactivity are even closer for isocyanide
and vinylidene complexes, however. The method discovered
by Chatt and co-workers for the preparation of metal
alkoxy(amino)- and diaminocarbene compounds (see Section 4), which is based on the nucleophilic attack of a Lewis
base such as ROH or RNH, on the carbon atom of the
i s o ~ y a n i d e , [can
~ ~ I readily be extended to vinylidene complexes, in particular when these are cationic. The cl-carbon
atom of the vinylidene ligand is then even more electrophilic
than in comparable neutral compounds, so that, for example, starting from [CpFe(C = CH2)(CO)(PPh3)J@
and alcohols, amines, mercaptans or hydrogen halides, it is possible
to obtain various types of iron carbene complexes. Figure 18
contains a summary of the results described by Hughes et
form the square-planar alkyne compounds trans-[RhCl(RC= CH)(PiPr,),] into the octahedral alkynyl(hydrid0)
complexes [RhHCl(C E CR)(PiPr,),(py)] by adding pyridine
and to treat these with CpNa to give the half-sandwich compounds [CpRh(C = CHR)(PiPr,)];"
a little later, using
iridium as the central atom, we were able to synthesize the
complete series of isomers shown in Scheme 6." 'I According to more recent studies, there is no doubt that an
analogous stepwise conversion of HC =CR to C = CHR also
occurs for rhodium.['02, 'I
[ Fe] =C
[ Fe]=C
[Fe] =
Fig. 18. Reaction of [CpFe(C =CH,)(CO)(PPh,)lm with nucleophiles
Another feature common to the parent compound CO
and its near relatives CS and C = CH, should be mentioned.
Since the beginning of this century it has been known that
metal carbonyls react with thermally or photochemically
generated 16-electron complex fragments to give binuclear
compounds in which the CO acts as a bridge; the prototype
Scheme 6. L = PiPr,. Middle: 6(IrH) % -43.
of this reaction is the formation of [Fe2(C0),] from [Fe(CO),] and [Fe(CO),] . [ l Z 3 ] Using the isostructural series
[CpRh(CX)(PR,)] with X = 0, S and CH, as an example,
These results seem at first to be in complete disagreement
we were able to show that these three complexes react in
with the MO calculations of Silvestre and H o f f m ~ n n . ~ ~ ~ ~ ]
exactly the same way with the 16-electron species [CpMnHowever, these authors started from complex building
(CO),] generated by irradiation of [CpMn(CO),] and stabiblocks [ML,] such as [CpMn(CO),] (or more simply
lized by T H F (Fig. 19).L65*
1241 It is remarkable that not only
[MnHJ4@)in which the metal has a 16-electron configuration; thus, addition of the alkyne gives a filled shell with 18
electrons. This requirement is not met, however, by the fragment [MCl(PiPr,),] (M = Rh, Ir): here, the metal has only a
14-electron configuration, so that a free coordination site is
still present after addition of the alkyne. For this reason the
hydride transfer from the metal to the 8-C atom of the
X = 0 ,S , CH2
alkynyl ligand in the intermediate [MHCI(C = CR)(PiPr,),]
could occur intermolecularly ; this is in agreement with preFig. 19. Reaction of the isostructural compounds [CpRh(CX)(PR,)] with
liminary kinetic data (for the example M = Ir).[1191
Angew. Chem. Ini. Ed. Engl. 29 (1990) 1077-1089
the CO but also the CS and C = C H , bridges occupy an
unsymmetrical position between the two metal atoms; this is
presumably due to the dissimilar electron configurations at
rhodium and manganese.
6. On to the Second Century:
The Search for the Missing Relatives
What does the future look like? Will the organometallic
family which is now celebrating its birthday increase in size,
and in which direction could this perhaps take place? Which
will be the next metal-bonded relative of CO?
One very promising candidate is certainly SiO. On the
basis of earlier work by Schmid et al.,[1251Zybill et al. have
very recently shown that dichlorosilylene, SiCl,, can be generated at a 16-electron complex fragment just as can a carbene and that the M-SiC1, bond can be stabilized by means
of adduct formation with suitable Lewis bases such as hexamethylphosphoric triamide (HMPT)."
The X-ray structural analyses of [(CO),CrSiCl,(hmpt)] and [(CO),FeSiCl,(hmpt)] show that the M-Si distance is relatively short and
that the HMPT molecule is only weakly
If it
were possible to displace the HMPT molecule and replace it
by H,O and thus to eliminate two molecules of HCI, one
could generate the CO analogue SiO. A very similar conversion of CC1, coordinated at osmium to CO has been described by Roper and c o - w o r k e r ~ . [In
~ ~order
to fix the silicon monoxide ligand it may be necessary to use an electronricher complex building block such as [C,Me,Re(PR,),],
[Os(PMe,),], or [C,Me,Ir(PMe,)] rather than [Cr(CO),] or
[Fe(CO),]; such building blocks could stabilize the M-SiO
bond by means of stronger 71 back-bonding.
If it were possible to fix SiO, then why not also BO@?In
Section 2, we referred to the existence of the extremely shortlived species CH,BO, which is isoelectronic with BH,CO;
the former can be thought to be built up from CHF and
BOO. According to H ~ f S m a n n , [ ' ~CHF
is isolobal with
[Mn(CO),]@ and [Co(CO),]@, so that compounds such as
[Mn(CO),(BO)] and [Co(CO),(BO)] (i.e., analogues of [Cr(CO),] and [Fe(CO),]) would be plausible synthetic goals.
Attempts to synthesize these complexes could build on the
work of Noth and Schmid, who reported the preparation of
the readily decomposing compound [(CO),CoBCl,] as early
as 1966." Addition of a Lewis base such as HMPT would
give an intermediate similar to that formulated above for
Six,. Careful protolysis could then convert an ElX, ligand
into an El0 ligand (El = B). Boron compounds with a coordination number of two are no longer curiosities today, as
has been shown clearly in the last decade, in particular by
Noth, Parry, Paetzold, and Berndt." "I
Angen. Chem. lnt. Ed. Engl. 29 (1990) 1077-1089
What other possibilities are there apart from SiO and
BOO? What is the situation regarding CFB, which we have
already mentioned in Scheme 2, or BF, which is also isoelectronic with CO? Is it realistic to think of such ligands becoming available? If intuition is not deceiving, a compound of
the type L,M(CF) will soon be described in the literature.
The question will then be only whether this molecule is better
described as a carbyne complex [L,M = C - F] than as a carbony1 analogue [L,M = C = F]; this question can only be
answered by an X-ray structural analysis, together with MO
calculations. The signposts on the way to [L,M(CF)] have
probably already been set up by Roper, who, during the
course of extremely interesting studies, has been able to
synthesize several compounds containing the fragments
M(CC1,) and M(CF,).[' 301 Although compounds such as
[RuCl,(CF,)(CO)(PPh,)] and [Os(CF,)(CO),(PPh,),] have
not yet led to products containing an OsCF
compound [OsCl(CPh)(CO)(PPh,),] obtained by the same
group['321shows that the goal of synthesizing a neutral complex [OsCl(CF)(CO)(PPh,),] or a cation [Os(CF)(CO),(PPh,),]@ is quite realistic. The existence of trinuclear clusters with p,-CF bridges such as [((CO),CO),(~-CF)]['~~~
[((CO),Fe),(p,-CF),]['341is also encouraging in this respect. These complexes resemble the corresponding carbonyl
compounds with triply bridging CO ligands in both their
structure and their bonding features.
A complex such as [Fe(CO),(BF)] or [(C,R,)Os(L)(BF)]
also seems attainable. Schmid, Petz, and Noth [13'] described
compounds of the formula [Fe(CO),(BNR,)] (R = Me, Et)
as early as 1970; while these are extremely thermally labile,
there is no doubt that they do exist. Although the road
from [Fe(CO),(BNR,)] to [Fe(CO),(BF)] will not be an easy
one, it does appear passable. Reactions at osmium also
appear feasible; the building blocks [C,H,Os(PR,)] ,I1
[C,H,OS(CO)],~'~'~and [(C6H3Me3)O~(CO)],[1381
used by
us, could well prove suitable for the attachment of a BF
One hundred years have passed and a new era lies before
us. One does not really need to be a prophet to forecast that
the second century will also be an exciting one, that further
surprises await us, and that the present survey will certainly
not be the last occasion on which the serendipitous discovery
made by Ludwig Mond will be remembered with reverence.
I thank Heinrich Vahrenkamp for suggesting this subject
and Ernst Otto Fischer for valuable information regarding the
material to be consulted. Oliver Niirnberg helped in the preparation of the reaction schemes, figures, etc., and I thank him
heartily for this help.
Received: February 14, 1990 [A 781 IE]
German version. Angew. Chem. 102 (1990) 1109
Translated by Prof. 7:N . Mitchell. Dortmund
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